Rate-responsive pacemaker

ABSTRACT

A pacemaker has means for producing first and second signals one of which changes rapidly in response to changes in exercise level but may inaccurately represent the appropriate pacing rate. Such signal may be derived from a vibration sensor. The second signal accurately represents the required pacing rate but changes slowly in response to exercise level and may represent any of a number of different physiological variables, such as respiratory rate. The signals may be derived from the same or separate sensors. A control circuit provides a pacing rate appropriate to resting conditions if the first signal has a value less than a threshold regardless of the value of the second signal. The pacing rate is elevated to a predetermined level, such as 95 beats/min, if the first signal exceeds the threshold but the second signal has a value indicative of a pacing rate less than the predetermined level. The pacing rate is further elevated to substantially equal that indicated by the value of the second signal when the first signal exceeds the threshold and the second signal indicates a rate higher than the predetermined rate.

BACKGROUND OF THE INVENTION

The present invention relates to a cardiac pacemaker.

In the normal healthy individual the output of blood by the heart isvaried continuously to meet the metabolic demands of the body. Thecardiac output changes with variations in the heart rate and the volumeof blood pumped at each heart beat. The cardiac output is the product ofthese two variables. During strenuous exercise a four or five foldincrease in cardiac output may be required. Approximately 75% of thisincrease is achieved by elevation of the heart rate.

Patients with cardiac disease resulting in very slow heart ratesfrequently require treatment in the form of an implanted artificialpacemaker. This consists of a small electrical pulse generator that isconnected to the heart by an insulated electrode lead, usuallypositioned in the right ventricle.

Until recently the majority of such pacemakers were relatively simpledevices that provided a constant frequency of stimulation. These sufferfrom the disadvantage that they are unable to increase the heart rateduring exercise, thereby limiting the degree of exertion that thepatient is able to undertake. This drawback has led to the developmentof two forms of pacemaker that can increase stimulation rate duringexercise. The "ideal" system is a dual chamber pacemaker which has asecond lead implanted in the right atrial chamber of the heart. Thisdetects the rate of the heart's natural pacemaker and enables theartificial pacemaker to provide normal physiological changes in heartrate. However, such systems are not suitable for many patients and alsohave inherent technical and economic disadvantages.

An alternative approach that has been developed during recent years isthe single chamber rate-responsive pacemaker. The fundamental principlesof this device are as follows:

(A) A sensor is used to detect changes in a biological variable that aredirectly or indirectly proportional to the level of metabolic activityin the body. Evoked QT interval, respiratory rate, mixed venoustemperature and body vibration are the main variables that are sensed bycurrently available rate-responsive pacemakers.

(B) The output from the sensor is processed by electronic circuitry toprovide a signal that can be used to vary the stimulation rate providedby the pacemaker. This circuitry is designed to perform a series ofmathematical and logical functions that are known collectively as therate-response algorithm.

The two principal functions of the rate-responsive algorithm are:

i) To determine the recognition level within the pacemaker for the valueof the sensor signal which must be exceeded in order for an increase inpacing rate to be initiated (threshold level).

ii) To determine the precise mathematical relationship between changesin the sensor signal S and the change in pacing rate P to be provided bythe pacemaker, (the slope dP/dS).

Although every individual requires substantially the same heart rate asother individuals for a given level or type of physical activity, thechanges which take place in the sensed biological variable as a resultof a given level of physical activity differ from individual toindividual. Accordingly, conventional rate response pacemakers arearranged so that the values of the threshold and slope may be adjustedto suit the individual patient. These values can be selected bytelemetered instructions from an external programmer. The programmer isalso used to select the minimum and maximum rates that the pacemakerwill generate and to vary a number of other aspects of pacemakerfunction such as pulse amplitude and duration.

Conventional rate responsive pacemakers utilise a single biologicalsensor to modulate the pacing rate. Studies on the clinical performanceof such pacemakers have revealed that no single sensor system is able toaccurately reproduce the changes in heart rate that are seen in healthyindividuals during the performance of normal daily activities. Thuspacing systems that detect movement of the body (e.g. by sensingphysical vibrations) are able to provide correctly timed changes inheart rate but do not necessarily provide a response that isproportional to the degree of exercise that the user is undertaking.Pacing systems that indirectly detect changes in metabolic activity(e.g. by sensing the rate of respiration or the temperature of mixedvenous blood) are able to generate changes in heart rate that are morephysiologically appropriate in terms of their magnitude, but theseresponses may take place so slowly that they are too late to be ofhaemodynamic benefit to the user.

European Patent Application 249820 discloses an attempt at solving theseproblems. The pacemaker disclosed in that document includes severalsensors each for sensing a respective different physiological variableand a selector circuit which successively selects different ones of thesignals and/or combinations thereof in predetermined time steps afterthe start of an exercise cycle. In the example given, an accelerationsensor determines the start of an exercise period and controls thepacing rate during the initial portion of the exercise period which ispredetermined and selected to be within the range 10 to 60 seconds.During the middle period of the exercise, which is predetermined andselected to be within the range 30 seconds to 20 minutes, a respirationsensor determines pacing rate. In the final period of the exercise whichis also predetermined and selected to be within the range 30 seconds to3 minutes, pacing rate continues to be determined by the respirationsensor. Although the European patent application also indicates that theselected signal and/or combination of signals may also be dependent uponother factors such as predetermined pacing rates, those output signalswhich fluctuate least, or the falling below a threshold of a singleoutput signal or combination thereof, there is no specific descriptionof what these dependencies might be in practice.

The present inventors consider that a pacemaker constructed as describedin European Application 249820 would, in practice, suffer fromdisadvantages. For example, during the initial predetermined time periodof the exercise, namely from 10 to 60 seconds, pacing rate varies withthe magnitude of the signal from the accelerometer and thus, during thisperiod, due to the deficiencies in such sensors, the actual pacing ratemay differ substantially from the most appropriate pacing rate. Forexample, a person running downstairs may produce substantial vibrationwhich would cause the accelerometer to produce a large output signalwhich in turn would cause the pacemaker to produce a higher pacing ratethan actually required whereas a person running upstairs may produceless vibration, resulting in a lower pacing rate than actually required.Further, the predetermining of the lengths of the time periods in anexercise cycle during which different sensor signals are selected willmean that, in practice, where an individual undertakes a variety ofdifferent exercise cycles, the switching from one sensor signal toanother may take place at inappropriate times.

Hertzschrittmacher, 6, 1986 pages 64 to 67 contains a report of somestudies carried out into the possibility of providing a pacemaker withtwo sensors, one sensing bodily vibrations and the other central venousblood temperature. This study proposes that the advantages of the twosystems can be obtained by utilizing the combination of such sensors andproposes that the pacing frequency increase should result from either ablood temperature increase or a change in the measured activity. Ineffect, therefore, this system would select a pacing rate equal to thehigher of the rates indicated by the two sensors. In view of theinaccurate relationship between the magnitude of the signal output by avibration sensor and required pacing rate, this system would not besatsifactory.

EP 259658 (Intermedics Inc) discloses a further attempt at solving theproblem that vibration sensors, whilst responding rapidly to bodilyvibrations, do not provide an accurate indication of the pacing raterequired whereas sensors which sense such changes in metabolic activityas rate of respiration or mixed venous blood temperature respond slowlybut, after response, may provide a signal which accurately indicates anappropriate pacing rate. EP 259658 suggests that filtering the output ofa vibration sensor to suppress frequencies above approximately 4 Hz willprovide a signal which not only responds rapidly to changes in bodilyvibration but also has a "direct and substantially linear relationship"with the work being performed by the patient. The pacing rate isdetermined by an algorithm which utilises such a signal and a signalfrom a second sensor sensing, for example, blood temperature. Thisalgorithm involves establishing a number of successively higherthresholds for the amplitude of the filtered signal from the vibrationsensor and a number of fixed predetermined successively higher pacingrates associated with each threshold respectively. When the amplitude ofthe filtered signal from the vibration sensor exceeds a given threshold,the pacing rate is increased (at a predetermined rate of increase) tothe respective value and counting of a predetermined time-out period isbegun. If the signal from the second sensor (e.g. blood temperaturesensor) increases within the time-out period to a level indicating thatthe required pacing rate is higher than the predetermined pacing rateassociated with the relevant threshold, further increase in the pacingrate takes place in accordance with the level of the signal from thesecond sensor. If, however, the signal from the second sensor fails toincrease to such a level within the time-out period, it is assumed thatthe increase in the filtered vibration sensor signal has been in errorand a fault recovery program is executed to reduce the pacing rate backto a base level.

The algorithm disclosed in EP 259658 is also such that if the signalfrom the second sensor increases without the amplitude of the filteredsignal from the first sensor having exceeded its first threshold someincrease in pacing rate may take place under control of the signal fromthe second sensor, but this increase is limited to a small value. Thedescription in EP 259658 appears to envisage that, under conditionswhere the amplitude of the filtered signal from the first sensor hasexceeded a given one of the thresholds, the further increase in pacingrate which may take place under control of the second signal (providedit has increased sufficiently within the time out period) is limited toa small value so that the higher pacing rates can only be achieved ifthe filtered vibration sensor signal first exceeds successively higherthresholds.

Regardless of whether filtering out frequencies above 4 Hz in a signalderived from a vibration sensor would actually provide a signalsubstantially linearly related to the work being performed by a patientas suggested in EP 259658, the proposal in this patent suffers from anumber of problems.

Firstly, the setting up of even a single threshold is a relativelycomplicated and skilled operation requiring the patient to undergoselected exercises whilst a skilled technician or physician monitors theoperation of the pacemaker in order to enable him to program in anappropriate threshold. The setting of a number of successively higherthresholds, therefore, would be extremely complex and time consuming andthe present inventors believe that it may in fact be impossible toachieve a satisfactory set of such thresholds in the majority ofpatients. Further complexity arises in the need for setting time-outperiods and the need for defining the small amounts of increase inpacing rate which may take place if the second signal increases withoutthe amplitude of the filtered vibration sensor signal having firstexceeded an appropriate threshold.

Secondly, the algorithm disclosed in EP 259658 is such that a number ofcommon activities could not be adequately provided for. One example isany form of exercise that generates significant body vibrations but doesnot require high levels of energy expenditure, e.g. brushing one'steeth. Such a form of exercise will generate high amplitude vibrationsin the frequency range of 1-4 Hz. These will be detected by the activitysensor and cause a rapid rise in pacing rate as a number of vibrationthreshold levels are exceeded. The complementary sensor will notindicate the need for a pacemaker rate response and will eventuallyensure that the pacemaker rate returns to near base levels. However,this decline in rate will not occur until the termination of the timeout period, and will thus not prevent a period of excessive pacing rateoccurring which may last for two or three minutes, for example. Anotherexample is any form of exercise that requires a high level of energyexpenditure by the patient but does not generate high amplitudevibration signals, e.g. swimming. In this situation it is likely thatonly a lower vibration threshold level will be exceeded, resulting in asmall increase in pacemaker rate. The rate will of course increasefurther during sustained exertion when the ongoing energy expenditure isdetected by the complementary sensor, but it appears that the algorithmof EP 259658 will limit the magnitude of this secondary rise inpacemaker rate if the vibration signal remains small.

SUMMARY OF THE INVENTION

The invention, at least in its preferred embodiment, aims to solve theabove problems. The way in which this solution is achieved by theinvention will be understood by considering the following description.

In a preferred aspect, the present invention provides a rate responsivepacemaker that incorporates two distinct modalities of biologicalsensing. These two modalities may be detected by means of two separatesensor systems or they may be derived by electronic processing of thesignal from a single sensor. In order to simplify the description of thepresent invention it will be assumed that two separate sensors arerequired. Signals from the two sensors will be analysed simultaneouslyby a dual-channel rate response algorithm. This algorithm is designed toretain the beneficial aspects of the responses provided by eachdifferent sensory modality while at the same time minimising theiradverse aspects.

The two sensors will be termed Sensor 1 and Sensor 2. It must beemphasised that Sensor 1 can in practice be any sensor that provides animmediate response to a change in the level of physical exertion beingperformed by the user whereas Sensor 2 can be any sensor that accuratelydetermines the magnitude of this change. However, for the purposes ofthe following description, it will be assumed that Sensor 1 detects bodyvibration and Sensor 2 measures the rate of respiration.

Sensor 1 serves to detect whether or not the user is undertakingphysical exertion of sufficient magnitude to necessitate an increase inheat rate. During a sustained period of physical inactivity, the lack ofa signal from Sensor 1 will keep the entire rate response algorithm in a"deactivated" state, thereby allowing the pacemaker to function "ondemand" at a preset minimum rate. In this state, signals from Sensor 2will be ignored and will thus be incapable of initiating "unwanted"increases in pacing rate (e.g. as may occur with hyperventilation in theabsence of physical exertion).

With the onset of physical exertion, however, Sensor 1 will detect theincreased amplitude and magnitude of body vibrations and when theseexceed a preset threshold value, the rate response algorithm will beactivated. A first channel (Channel 1) of the algorithm will initiate anincrease in pacing rate. This response will be of an "all or none"character in that the pacing rate will rise progressively until apredetermined upper rate limit is reached. For the purposes of thisdocument it will be assumed that the response will take the form of alinear increase in rate with a gradient of 1 beat/min/sec. The restingpacing rate may be 70 beats/min and the upper rate limit shouldpreferably be at least 20 beats/min higher than the resting rate andpreferably not more than 30 beats/min higher than the resting rate. Itis particularly preferred that the upper rate limit be 25 beats/minhigher than the resting rate, namely 95 beats/min when the resting rateis 70 beats/min. It should be understood, however, that these values aregiven by way of example. It should, however, be understood that aparticularly preferred aspect of the invention involves the provision inchannel 1 of a single threshold, which threshold is selected torepresent an exercise level requiring an increase of at least 20beats/min relative to the resting rate.

The pacing rate will remain elevated for as long as sensor 1 continuesto detect supra-threshold body vibrations. As described above, thehighest pacing rate that can occur solely as a result of activation ofSensor 1 is, say, 95 beats/min. A provision of the current invention isthat the signals detected by Sensor 2, in effect, serve to modulate thisupper rate limit.

The output from Sensor 2 will be fed to a second channel (Channel 2) ofthe rate response algorithm. This channel will incorporate circuitrythat can assess the rate and/or depth of respiration and therebydetermine a pacing rate that is physiologically appropriate for thelevel of exercise being undertaken by the user. If the signals fromSensor 2 are such that a rate of less than the upper rate limit ofChannel 1, e.g. 95 beats/min is indicated, the pacemaker will continueto function at the rate (e.g. 95 beats/min) as determined by Channel 1of the rate response algorithm. If the signal from Sensor 2 indicates arate that is higher than the Channel 1 maximum (e.g. 95 beats/min),however, then this rate will be adopted by Channel 1 as a revised upperrate limit. The pacemaker rate will rise progressively until this newupper limit is attained. A programmable "maximum upper rate" value willbe employed to ensure that the pacing rate cannot rise to levels thatwould adversely influence cardiovascular function and thereby havedeleterious consequences for the user.

A drop in the level of exercise will be detected by Sensor 2 and thiswill in turn lead to a lower rate being generated by Channel 2 of thealgorithm. This will reduce the upper rate limit for Channel 1 and acorresponding drop in pacing rate will follow. If the level of exercisedeclines to such an extent that the vibration signals detected by Sensor1 fall below the preset threshold, the entire algorithm will return tothe "deactivated" state. In this state, the pacing rate will fallprogressively until the minimum rate of 70 beats/min is reached. It isenvisaged that a constant rate of decline of pacing rate will beprovided under these circumstances, and a possible value for this rateof decline would be 0.5 beat/min/sec. It therefore follows that thehigher the pacing rate is at the moment of cessation of exercise, thelonger it will take for the pacing rate to decline to the resting level.Applicants also acknowledge the disclosure of European PatentApplication 191404 which discloses a pacemaker utilising an activitysensor (i.e. a vibration sensor) and a physiological sensor. In order toconserve the battery, the physiological sensor and associated circuitryis normally turned completely off. However, if the activity sensorproduces a signal higher than a given threshold, then the physiologicalsensor and associated circuitry is turned on and pacing rate controlled,at all times, in accordance with the magnitude of the signal from thephysiological sensor. The sole object of the arrangement disclosed inEuropean Application 191404 is to conserve the battery and this documentdoes not address the problems with which the present invention isconcerned.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described further, by way of example, with reference tothe accompanying drawings, in which:

FIG. 1 is a functional block diagram illustrating a preferred embodimentof the present invention;

FIG. 2 is a block diagram showing part of the embodiment of FIG. 1 inmore detail;

FIG. 3 is a flow chart showing the functions performed in the preferredembodiment of the invention;

FIGS. 4 and 5 are graphs illustrating operation of the preferredembodiment of the invention; and

FIG. 6 is a block diagram showing a modification to the preferredembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The illustrative implantable rate responsive pacemaker shown in FIG. 1comprises a hermetically sealed can 1 containing a power source 2 (e.g.a battery), an activity sensor 3, an activity sensor processor 4,circuitry 5 and 6 defining two channels of a rate response algorithm,and stimulating circuitry 7. A lead 8 extends through the wall of thecan to a pacing electrode (not shown) for applying an electricalstimulus to the heart of the user of the pacemaker. A respiratory sensor9 is situated outside the can and is connected to a respiratory sensorprocessor 10 within the pacemaker.

The activity sensor 3 is sensitive to the level of physical vibrationwithin the user's body. It generates an activity signal that isamplified and processed by processing circuitry 4 before being passed tocircuitry 5 that constitutes channel 1 of the rate response algorithm.Under resting conditions, this channel of the algorithm defines aconstant minimum pacing rate by means of a pacing control signal that isapplied to the stimulation circuitry 7. The value of this minimum pacingrate, and other required values to be described below, can be selectedby means of a telemetry signal from an external programming unit 15.Thus, the can 1 contains a telemetry receiver and transmitter device 16having an antenna 17 so that information may be exchanged with theexternal programmer 15. A reed switch 11 is provided in the can 1 foractivating the device 16 in response to the proximity of a magnet 12provided in the external programmer 15.

When the user performs physical exercise, the amplitude and frequency ofthe activity signal will increase. When the amplitude and/or frequencyof the signal exceeds a preset threshold value, channel 1 of the rateresponse algorithm will respond by increasing the rate of the pacingcontrol signal. The activity signal threshold value may be selected bymeans of a telemetry signal from the external programming unit.

The resulting increase in the rate of the pacing control signal will bein the form of an "all or none" response. The rate will riseprogressively from the preset minimum value until a preset upper ratelimit is reached (for example, the rate will rise linearly by 1beat/min/sec from 70 beats/min up to 95 beats/min). Having reached theupper rate limit, the pacing control signal of Channel 1 will remain atthis level for as long as the activity signal remains above itsthreshold value.

If the level of physical exertion being undertaken by the user declinesto such an extent that the activity signal falls below its thresholdvalue, the entire rate response algorithm will return to a "deactivated"state and the rate of the pacing control signal will fall progressivelyto the preset minimum value (for example, it will decline linearly by0.5 beats/min/sec until a rate of 70 beats/min is reached).

The respiratory sensor 9 detects changes in the electrical impedance ofthe user's chest wall. Chest wall impedance varies with respiratorymovement, and the user's respiratory rate at any given moment isrepresented by the number of cyclical variations in impedance that areoccurring per minute. The signal representing these impedance changes isamplified and processed by sensor processor 4 before being analysed inchannel 2 of the rate response algorithm. If the amplitude of thisimpedance signal falls below a predetermined threshold level, it will berejected by the rate response algorithm. Under these circumstances, theupper rate limit of channel 1 of the algorithm will remain unchanged.

However, if the amplitude of the impedance signal exceeds the thresholdvalue, channel 2 of the algorithm will respond and increase its outputrate. The mathematical relationship between the change in the frequencyof the respiratory sensor signal (and thus a change in the rate ofrespiration) and the subsequent change in algorithm output rate is setby a predetermined slope value. Values for both the respiratorythreshold and the respiratory slope can be selected using telemetrysignals from the external programming unit. In clinical use, values willbe selected by the physician so that channel 2 of the algorithm willproduce an output rate that is physiologically appropriate for theuser's rate of respiration (and hence appropriate for the level ofphysical exercise he is undertaking).

If the channel 2 output rate is less than the initial upper rate limitof channel 1 of the algorithm (for example, less than 95 beats/min),then the upper rate limit of channel 1 will remain unchanged. If thechannel 2 output rate exceeds this value, however, then the upper ratelimit of channel 1 will be raised accordingly. Thus, in effect, theupper rate limit of channel 1 of the rate response algorithm ismodulated by the output of channel 2 of the algorithm. There will be aceiling rate that the upper rate limit cannot exceed, and this valuewill be selected by a telemetry signal from the external programmer.

The pacing control signal produced by channel 1 of the rate responsealgorithm is fed to conventional pacemaker stimulation circuitry 7.Sensing circuitry 7a is employed to monitor the user's intrinsic cardiacbehaviour. If the user's heart beats spontaneously at a rate higher thanthat determined by the pacing control signal, the output of thepacemaker stimulation circuitry will be inhibited. The pacemaker willthus function "on demand" in that it will only stimulate the user'sheart to beat if the inherent natural heart rate falls below thephysiologically appropriate rate (as determined by the rate responsealgorithm).

Stimulation circuitry 7 is employed to deliver impulses of predeterminedamplitude and duration to the user's heart via a conventional unipolaror bipolar transvenous pacing lead 8. The rate at which these impulsesare delivered is determined by the pacing control signal and the timingof impulse delivery may be influenced by the pacemaker sensingcircuitry.

The rate response algorithm circuitry 5, 6 may be switched into apermanently "deactivated" state by means of a telemetered instructionfrom the external programming unit 15. In this state the incomingsignals from both biological sensors will be ignored and the pacemakerwill continue to function "on demand" at the preset minimum pacing rate.

The telemetry reception/transmission circuitry 16 may be conventionaland will be used to control the following aspects of pacemaker function:

(a) Mode, namely "Demand, Fixed Rate" or "Demand, Rate Responsive".

(b) Minimum pacing rate--for both modes.

(c) Maximum pacing rate--for "Rate Response" mode only.

(d) Activity sensor threshold value.

(e) Respiratory sensor threshold value.

(f) Respiratory slope value.

(g) Pacemaker stimulation and sensing parameters--including stimulusamplitude and duration, refractory period, and sensitivity.

(h) Telemetry function--to enable the pacemaker to transmit the valuesof all programmable parameters out to the external programmer.

FIG. 2 illustrates in more detail the circuitry enclosed within brokenline 20 of FIG. 1, in accordance with a preferred embodiment of theinvention. As shown in FIG. 2, the signal from the activity sensor 3,after processing in circuitry 4, is applied to circuitry 21 which may bein accordance with U.S. Pat. No. 4,428,378 (Anderson et al), thecontents of which are incorporated herein by reference, and thus maycomprise a band pass amplifier 22, level detector 24 and zero crossingdetector 26. If the amplitude of the signal supplied by the amplifier 22to the level detector 24 exceeds a threshold defined by a signalsupplied on line 27 to circuitry 21 from the telemetry receiver andtransmitter circuitry 16, zero crossing detector 26 outputs a series ofpulses at a frequency indicative of the level of activity sensed by theactivity sensor 3. These output pulses are supplied to a rate-responselogic and control circuit 28.

The signal from respiratory sensor 9 and processing circuitry 10 isapplied to circuitry 30 defining the respiration rate responsealgorithm. This circuitry may be in accordance with U.S. Pat. No.4,567,892, the contents of which are incorporated herein by reference,and has stored therein respiratory threshold and slope values receivedfrom the telemetry device 9. The output of circuitry 30 is also suppliedto the rate-responsive logic and control circuit 28.

Circuit 28 has stored therein values for minimum and maximum pacing ratereceived from the telemetry device 16 and is operative to perform theabove described functions. These functions may be provided by hardwareor software or a combination of both but, in the present embodiment, itwill be assumed that circuit 28 includes a microprocessor which isprogrammed to carry out the required functions in accordance with theflow chart shown in FIG. 3.

In FIG. 3, the frequency of the signals from circuit 21 is representedby A, and T represents the threshold frequency thereof which defines thelevel of activity above which pacing rate will be increased. The actualcurrent pacing rate is represented by P and the signal output by circuit30, representing the pacing rate indicated by the respiratory sensor, isrepresented by R.

At step 1 in FIG. 3, the microprocessor determines whether the frequencyof activity signal A is greater than the threshold T. In thisembodiment, this determination is preferably made by counting the numberof pulses received by circuitry 28 from zero--crossing detector 26 in apreselected period of time. In this way, circuitry 28 determines whetherthe frequency of the pulses output by sensor 4 having an amplitude abovethe level set by level detector 24 is greater than the selectedthreshold frequency T. If A is less than T, step 2 determines whetherthe pacing rate is greater than the minimum pacing rate stored incircuitry 28. If it is not, the program returns to step 1 withoutchanging the pacing rate. If it is, the program moves to step 3 whichinitiates a decrease in the pacing rate, at a fixed rate such as 0.5beats/min/sec. Following this initiation, the program returns to step 1and the decrease in P continues, at the fixed rate, with the programmoving through steps 1, 2 and 3 until such time as step 2 indicates thatP is not greater than the minimum. At this time, the decrease is stoppedand the program loops to step 1.

If, at any time, step 1 indicates that A is greater than T, the programbranches to step 4 which determines whether the actual pacing rate isless than a predetermined level stored in circuitry 28 and indicated asbeing 95 beats/min. If step 4 indicates that P is less than 95beats/min, the program moves to step 5 which initiates an increase in Pat a fixed rate, such as 1 beat/min/sec. Following this initiation, theprogram loops back to step 1 and continues to move through steps 1, 4and 5 until such time as step 4 indicates that P is not less than 95. Atthis time, the program branches to step 6 which determines whether theactual pacing rate is greater than the rate indicated by the respiratorysensor and circuitry 30. Normally, at the beginning of a period ofexercise, there will be some delay before signal R increases beyond thelevel indicative of 95 beats/min so that, initially, it will be expectedthat step 6 will result in a "yes" and the program will move to step 6Awhich determines whether P is greater than 95 beats/min. As P will nothave risen above 95 beats/min, this step will give a "no" and theprogram will continue to loop through steps 1, 4, 6 and 6A withoutfurther change of P. With sustained heavy exertion, R will increaseuntil it exceeds 95 beats/min. When this occurs, step 6 will result in a"no", and the program will move to step 7. Step 7 is included to ensurethat when P=R, the program will loop through steps 1, 4, 6 and 7 withoutchanging P. If R is greater than P, however, step 7 will result in a"yes". The program will move to step 8, where it is determined whether Pis less than its permitted maximum value as stored in circuitry 28. If Pis less than the maximum permitted value (as would be expected duringthe early stages of exercise), the program moves to step 5 whereby P isincreased at the above mentioned fixed rate.

The program will continue to move through steps 1, 4, 6, 7, 8 and 5 insequence until such time as steps 6 and 7 indicate that P is equal to R.When this condition occurs, the program will loop through steps 1, 4, 6and 7 without further increase and without decrease in the actual pacingrate. If the level of physical activity is further increased so that Rincreases, step 7 will result in a "yes" and, so long as step 8 does notindicate that P has reached its maximum permitted value, a furtherincrease in P, at the fixed rate above mentioned, will be initiated instep 5. If at any time, step 8 indicates that P has reached its maximumpermitted value, further increases in R would not result in any furtherincrease in P. If at any time, the level of physical activity decreasessuch that R becomes less than P, but does not decrease to an extent thatA becomes less than T, the program will branch from step 6 through step6A to step 3 so as to decrease the pacing rate P at the above mentionedfixed rate. This will continue until the rate P falls to 95 beats/min.At this time, step 6A will result in a "no" and step 3 will thereby beomitted from the loop, preventing a further decline in P.

The above sequence is continued until such time as the frequency A fallsbelow the threshold T and/or the circuit 21 ceases to output pulses, dueto the amplitude of the signal from amplifier 22 falling below thethreshold stored in level detector circuitry 24. As a result, eventhough R may remain at a high level, the program moves from step 1 tostep 2 at which it will be determined that P is greater than its minimumvalue and the program will then move to step 3, and loop through steps1, 2 and 3, so as to reduce the pacing rate at the above mentioned fixedrate of reduction until such time as step 2 indicates that P is notgreater than the minimum, whereafter the program will sequence throughsteps 1 and 2 until A again exceeds T.

It will be understood from the above description that pacing rate isonly increased if both the amplitude of the signals from the activitysensor 3 exceeds a threshold defined by the level detector circuit 24and the frequency of those signals exceeds the threshold frequency T.Thus, an advantage of the preferred embodiment of the invention is thatundesired pacing rate increases do not arise either from isolated or lowfrequency signals of relatively large amplitude from the activity sensor3 or from low amplitude relatively high frequency signals from thesensor 3.

It will be understood from consideration of the flow chart of FIG. 3 andthe foregoing description that, after the pacing rate has been increasedto the present level (95 beats/min in the example given) as a result ofA exceeding T, all further increase in pacing rate is dependent solelyupon the level of signal R derived from the second sensor. By thismeans, activities such as swimming may be adequately provided for sinceswimming may produce sufficient vibration for A to exceed T but wouldnot produce sufficient vibration for A to exceed a significantly higherthreshold. Further, activities such as brushing teeth producing highamplitude signals from the activity sensor would not result in anexcessive increase in heart rate, which would be undesirable, sincethere would be no appreciable increase in the signal R derived from thesecond sensor. However, in the proposal in EP 259658, the high amplitudesignals from the activity sensor would be expected to exceed severalthresholds and therefore cause an excessive and undesired increase inheart rate. Further, with the present invention, upon the rapid onset ofheavy exercise, the selected increase in pacing rate (25 beats/mingiving a pacing rate of 95 beats/min in the example) may be such as toenable the patient to cope with such exercise for a period long enoughto permit the signal from sensor 2 to catch up and then further increasethe pacing rate as required. Since, in the embodiment under discussion,there is no time-out period within which the signal from sensor 2 mustincrease to any particular level, there is no risk of deactivating thealgorithm with resultant reduction in heart rate as may arise in thearrangement disclosed in EP 259658 upon rapid onset of heavy exercisedue to the inclusion of the timeout period.

FIG. 4 illustrates graphically the operation of the pacemaker accordingto the preferred embodiment as so far described and, by way of anexample, illustrates a 10 minute period involving both periods ofactivity and of rest.

As shown in curve I of FIG. 4, the patient undergoes a relatively lowlevel of exercise during the first minute, rests during the next twominutes, undergoes a much higher level of exercise during the followingthree minutes and thereafter returns to a condition of rest.

As shown by curve II, the level of exercise in each period is sufficientfor the signal from the activity sensor and associated circuitry toexceed its threshold level.

There is a delay following the beginning of an exercise period beforethe patient's rate of respiration increases. Curve III shows that thefirst period of exercise, during which the exercise level is relativelylow, is not long enough to cause the respiration rate to increase at allbut the second period of exercise causes the respiration rate toincrease, the increase beginning after a delay of about three-quartersof a minute from the beginning of the period of exercise.

Curve IV illustrates the pacing rate which the output of circuitry 30indicates as appropriate. In view of the above mentioned delay,circuitry 30 does not indicate any increase in pacing rate during thefirst period of exercise and thus, if this alone were relied upon tocontrol the pacemaker, no increase in heart rate at this time wouldresult. This would be disadvantageous. Similarly, curve IV indicatesthat there is delay of about three-quarters of a minute after thebeginning of the second period of exercise before circuitry 30 begins toindicate that the heart rate should be increased. Again, if the heartrate were not to be increased during this delay period, that would bedisadvantageous.

Curve V shows that, despite the absence of a signal from circuitry 30indicating that the heart rate should be increased during the firstperiod of exercise, the pacing rate is nevertheless increased at a fixedrate as indicated at reference number 40. This increase arises from theprogram sequencing through steps 1, 4 and 5 illustrated in FIG. 3. Asindicated by reference number 42 in curve 5, the pacing rate increasesto a maximum level of 95 beats/min during the first period of exercise,this being achieved by the program sequencing through steps 1, 4, 6 and6A.

Reference number 44 in curve V indicates that after the activity signalhas fallen below the threshold, the pacing rate is reduced at the abovementioned fixed rate. This is achieved by the program sequencing throughsteps 1, 2 and 3 until the minimum rate of 70 beats/min is reached.

Reference number 46 in curve V shows that immediately the second periodof activity begins, the pacing rate increases at the above mentionedfixed rate to 95 beats/min, as shown by reference number 48 and remainsat that rate until there has been sufficient time for signal R toincrease to a level indicating a rate greater than 95 beats/min, thistaking about 11/2 minutes in the example shown from the beginning of thesecond period of exercise.

Reference number 50 in curve V indicates that, after this point, thepacing rate P is increased, again at the same fixed rate as previouslydescribed, in response to the continuing increase in signal R. Duringthe period corresponding to reference number 50, the program of FIG. 3sequences through steps 1, 4, 6, 7, 8 and 5.

At some point, the signal R reaches the maximum value appropriate to thelevel of exercise being undertaken and, as indicated by reference number52 in curve V, the pacing rate P is maintained constant at this maximumvalue. This is achieved by the program shown in FIG. 3 sequencingthrough steps 1, 4, 6 and 7.

As shown by reference number 54 in curve V, immediately the period ofexercise terminates, the pacing rate begins to decrease at the abovementioned fixed rate since the activity signal drops below the thresholdand the program sequences through steps 1, 2 and 3. This decrease takesplace despite the fact that there is a delay before the respirationrate, and therefore signal R, decreases. Thus, the preferred embodimentof the invention provides a substantial advantage that the pacing ratebegins to decrease immediately the exercise terminates rather thanremaining at a high rate until such time as the respiration rate hasdecreased.

FIG. 5 is included to illustrate, with curves I to V which have the samemeaning as curves I to V of FIG. 4, that from time to time, although thepatient may not undergo any physical activity of significance so thatthere is no increase in the activity signal, respiration rate maynevertheless increase to a level such that the signal from circuitry 30indicates that the pacing rate should be increased. However, in thesecircumstances, pacing rate increase is inappropriate and, as shown inFIG. 5, it does not in fact take place in the preferred embodiment ofthe invention since the program will remain sequencing through steps 1and 2 until such time as the activity signal exceeds its threshold andregardless of the level of signal R.

Thus, in the invention the pacing rate is always substantially at therequired level, any changes required begin to take place without anyundesirable delay and the rate of change may be preselected andtherefore selected to be optimum for increasing and optimum fordecreasing rate. This combination of advantages is not achievable in theprior art.

In the modification shown in FIG. 6, the respiratory sensor 9 is omittedand instead a single activity or vibration sensor 3 is employed. Theoutput from this sensor is applied to a band pass amplifier 60 centredat 10 Hz to provide an activity signal which is then supplied to sensorprocessor 10 and is also applied to a low pass amplifier 62 having apass band of zero to 0.5 Hz to provide a signal representing respirationrate which is applied to processing circuit 10. The sensor 3 may be asdescribed in U.S. Pat. No. 4,428,378 and the circuitry shown in FIG. 6enables that single sensor to provide both the activity signal and therespiratory signal, any unwanted cardiac signal being rejected. Themodified embodiment illustrated in FIG. 6 is otherwise the same as theembodiment described with reference to FIGS. 1 to 5.

As will be appreciated, various modifications may be made within thescope of the invention. The various figures for increase or decrease ofrate and maximum and minimum rate are merely given by way ofillustration. For example, although the minimum rate has been given as70 beats/min, it would be possible within the scope of the invention toprovide for an extra reduction in rate to say 60 beats/min, where thepatient is sleeping, this condition, for example, being indicated by theabsence of any activity signal for a predetermined period of time. Thus,the reference to a minimum rate above should be understood as meaningthe normal minimum rate, such as 70 beats/min. It is also within thescope of the invention for other values between the limits of say 30 to100 beats/min to be chosen and telemetred into the pacemaker using theexternal programmer 15, the antenna 17 and receiving circuitry 16.

Further, although in the embodiment described with reference of theaccompanying drawings, it has been assumed that detection of thefrequency of signal A has been carried out by counting the number ofpulses generated by zero crossing detector 26 in a predetermined period,other methods are possible. For example, the pulses output by the bandpass amplifier could be integrated over a predetermined period andcompared to an appropriate threshold.

In the above description, it has been indicated that the band passamplifier 22 may pass signals within a band centred at 10 Hz. Typically,the bandwidth of this amplifier might be, say, 6 Hz whereby signals inthe range 7 Hz to 13 Hz would be passed although alternativearrangements are possible, for example a band width of 4 Hz wherebysignals in the range 8 to 12 Hz would be passed. It is not essentialthat the centre frequency of the band pass amplifier should be 10 Hzalthough this is a particularly preferred value. It might, for example,be lower or higher. The processing performed on the signal from activitysensor 4 should be such that the processed signal may be compared with athreshold which corresponds to a level of activity for which theappropriate pacing rate is preferably at least 20 beats per minutehigher than the resting rate, for example 20 to 30 beats per minutehigher than the resting rate and particularly preferably about 25 beatsper minute higher than the resting rate. Thus a particularly preferredcombination of features in accordance with the invention involves thedetection of signals from the vibration sensor 4 having an amplitudeabove a predetermined level and a frequency of at least 7 or 8 Hz, andpreferably 10 Hz, the circuitry causing an increase in heart rate ofbetween 20 and 30 beats per minute, preferably 25 beats per minute aboveresting rate in response to the detection of such signals, furtherincreases beyond this rate being under the sole control of the level ofthe signal from the second sensor.

The pacemaker according to the invention may be arranged for operationin any of a variety of different modes. For example, it may be used inthe so-called AAI mode in which both pacing and sensing take place inthe atrium and pacing pulses are inhibited if spontaneous activity issensed, in the so-called VVI mode in which both pacing and sensing takeplace in the ventricle and pacing pulses are inhibited when spontaneousactivity is sensed or the so-called DDD mode in which pacing and sensingtake place in both the atrium and the ventricle with inhibition of thepacing pulses or triggering taking place in accordance with sensedactivity, in a well-known manner.

Further, although in the preferred embodiment as described, the changein pacing rate takes place linearly with time, it is possible within thescope of the invention for the changes in pacing rate to take placenon-linearly and it is possible for the changes to be made continuouslyor step-wise.

We claim:
 1. A pacemaker comprising:sensing means for providing a first signal which changes relatively rapidly in response to changes in a patient's exercise level but may inaccurately represent the pacing rate appropriate to said exercise level, and a second signal that changes from a normal condition relatively slowly in response to changes in exercise level but which, after having changed from said normal condition, relatively accurately represents a required pacing rate; and control means for generating a pacing signal to control pacing rate in response to said sensing means, said pacing signal being operative to provide: (A) a relatively low pacing rate when said first signal has a value less than a threshold; (B) a predetermined pacing rate higher than said relatively low pacing rate when the first signal exceeds said threshold and the second signal has a value indicative of a pacing rate less than said predetermined pacing rate; and (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal exceeds said threshold and the second signal indicates a rate higher than said predetermined pacing rate, regardless of the amount by which said first signal may exceed said threshold.
 2. A pacemaker according to claim 1, wherein the pacing signal generated by said control means is operable to change the pacing rate linearly.
 3. A pacemaker according to claim 1, wherein the pacing signal generated by said control means is operable to change the pacing rate nonlinearly.
 4. A pacemaker according to claim 1, wherein the pacing signal generated by said control means is operable to change the pacing rate stepwise.
 5. A pacemaker according to claim 1, wherein said sensing means comprises a first sensor means for providing said first signal and a second sensor means for providing said second signal.
 6. A pacemaker according to claim 5, wherein said first sensor means is operable to sense bodily vibrations and said second sensor means is operable to sense a physiological variable.
 7. A pacemaker according to claim 6, wherein said second sensor means senses a physiological variable selected from evoked QT interval, respiratory rate, minute ventilation, mixed venous temperature, venous oxygen saturation, or rate of change of right ventricular pressure.
 8. A pacemaker according to claim 6, wherein said threshold is defined as both a magnitude and a frequency of said first signal.
 9. A pacemaker according to claim 1, wherein said sensing means comprises a single sensor and processing means for processing signals from said single sensor to produce said first and second signals respectively.
 10. A pacemaker according to claim 9, wherein said single sensor is a vibration sensor and said processing means comprises filtering means.
 11. A pacemaker according to claim 10, wherein said filtering means is operable to provide said first signal by passing signals having frequencies within a frequency band in the region of 10 Hz and to provide said second signal by passing frequencies less than about 0.5 Hz.
 12. A pacemaker according to claim 11, wherein said second signal represents respiratory rate.
 13. A pacemaker according to claim 1, wherein said predetermined pacing rate is approximately 95 beats/min.
 14. A pacemaker according to claim 1, wherein said control means is operable to define predetermined minimum and maximum values for said pacing rate and to maintain the pacing rate within said values regardless of the values of said first and second signals.
 15. A pacemaker comprising:sensing means for providing a first signal which changes relatively rapidly in response to changes in a patient's exercise level but may inaccurately represent the pacing rate appropriate to said exercise level, and a second signal that changes from a normal condition relatively slowly in response to changes in exercise level but which, after having changed from said normal condition, relatively accurately represents a required pacing rate; and control means for generating a pacing signal to control pacing rate in response to said sensing means, said pacing signal being operative to provide: (A) a relatively low pacing rate unless said first signal includes frequency components having an amplitude above a predetermined amplitude and a frequency above a threshold frequency of at least 7 Hz; (B) a predetermined pacing rate higher than said relatively low pacing rate when said first signal includes frequency components having an amplitude above said predetermined amplitude and a frequency above said threshold frequency and the second signal has a value indicative of a pacing rate less than said predetermined rate; and (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal exceeds said threshold and the second signal indicates a rate higher than said predetermined pacing rate.
 16. A pacemaker according to claim 15, wherein said threshold frequency is 10 cycles per second.
 17. A pacemaker comprising:sensing means for providing a first signal which changes relatively rapidly in response to changes in a patient's exercise level but may inaccurately represent the pacing rate appropriate to said exercise level, and a second signal that changes from a normal condition relatively slowly in response to changes in exercise level but which, after having changed from said normal condition, relatively accurately represents a required pacing rate; and control means for generating a pacing signal to control pacing rate in response to said sensing means, said pacing signal being operative to define, for said first signal, a single substantially constant threshold value, and to provide: (A) a relatively low pacing rate when said first signal has a value less than said threshold value; (B) a predetermined pacing rate higher than said relatively low rate when the first signal exceeds said threshold value and the second signal has a value indicative of a pacing rate less than said predetermined pacing rate; and (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal exceeds said threshold and the second signal indicates a rate higher than said predetermined pacing rate.
 18. A pacemaker comprising:sensing means for providing a first signal which changes relatively rapidly in response to changes in a patient's exercise level but may inaccurately represent the pacing rate appropriate to said exercise level, and a second signal that changes from a normal condition relatively slowly in response to changes in exercise level but which, after having changed from said normal condition, relatively accurately represents a required pacing rate; and control means for generating a pacing signal to control pacing rate in response to said sensing means, said pacing signal being operative to provide: (A) a relatively low pacing rate when said first signal has a value less than a threshold; (B) a predetermined pacing rate higher than said relatively low rate when the first signal exceeds said threshold and the second signal has a value indicative of a pacing rate less than said predetermined pacing rate; (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal exceeds said threshold and the second signal indicates a rate higher than said predetermined pacing rate; and said pacing signal being operative to increase said pacing rate above said predetermined pacing rate solely in response to increase in said second signal.
 19. A pacemaker according to claim 18, wherein said predetermined rate is at least 20 beats per minute above said relatively low pacing rate.
 20. A pacemaker according to claim 19, wherein said predetermined rate is between 20 and 30 beats per minute above said relatively low pacing rate.
 21. A pacemaker according to claim 20, wherein said predetermined rate is approximately 25 beats per minute above said relatively low pacing rate.
 22. A pacemaker comprising:first sensing means for providing a first signal in response to bodily vibrations arising from a patient's activity; second sensing means for providing a second signal which changes in response to a physiological parameter which varies in response to a patient's activity; and control means for generating a pacing signal to control pacing rate in response to said first and second sensing means, said pacing signal being operative to define for said first signal a threshold for components of said first signal having a frequency of at least 7 Hz, and to provide: (A) a relatively low pacing rate when said first signal is such that said threshold is not exceeded; (B) a predetermined pacing rate which is from 20 to 30 beats/min higher than said relatively low pacing rate when the first signal is such that said threshold is exceeded and the second signal has a value indicative of a pacing rate less than said predetermined pacing rate; and (C) a pacing rate substantially equal to that indicated by the value of said second signal when the first signal is such that said threshold is exceeded and the second signal indicates a rate higher than said predetermined rate; and said pacing signal being operative to increase said pacing rate above said predetermined pacing rate solely in response to increase in said second signal. 